1. Field of the Invention
The present invention relates to a semiconductor memory device, a display and a portable electronic apparatus. More specifically, the present invention relates to a semiconductor memory device in which field-effect transistors each including a memory functional element having the function of retaining charges or polarization are arranged, and to a display and a portable electronic apparatus each having such a semiconductor memory device.
2. Description of the Related Art
Conventionally, a flash memory is typically used as a nonvolatile memory.
In a flash memory, as shown in FIG. 29, a floating gate 902, an insulating film 907 and a word line (control gate) 903 are formed in this order on a semiconductor substrate 901 via a gate insulating film. On both sides of the floating gate 902, a source line 904 and a bit line 905 are formed by a diffusion region, thereby configuring a memory cell. A device isolation region 906 is formed around the memory cell (see, for example, JP-A 05-304277 (1993)).
The memory cell stores information in accordance with an amount of charges in the floating gate 902. In a memory cell array configured by arranging memory cells, by selecting a specific word line and a specific bit line and applying a predetermined voltage, an operation of rewriting/reading a desired memory cell can be performed.
In such a flash memory, when the amount of charges in the floating gate changes, a drain current (Id)-gate voltage (Vg) characteristic as shown in FIG. 30 is exhibited. When the amount of negative charges in the floating gate increases, the threshold increases and the Id-Vg curve shifts almost in parallel in the Vg increasing direction.
In such a flash memory, however, the insulting film 907 which separates the floating gate 902 from the word line 903 is necessary from the viewpoint of functions and, in order to prevent leakage of charges from the floating gate 902, it is difficult to reduce the thickness of the gate insulating film. Consequently, it is difficult to effectively reduce the thickness of the insulating film 907 and the gate insulating film, and it disturbs reduction in the size of the memory cell.